Investigators are looking for two people whose images were captured before Monday's Boston Marathon terror attack and have distributed photos for “law enforcement eyes only,” according to multiple media reports.

The FBI had not released the photos of the two suspects to the media as of Thursday morning but had distributed them to law enforcement across the country.

The daunting task of sifting through thousands of images of the bombing site in search of a culprit has telescoped on Wednesday to a single video from a Lord & Taylor security camera.

The discovery of video of a man who wore a large backpack to the finish line area and then dropped the package there raised hopes for an imminent breakthrough in the case, setting off a media frenzy but insistent statements from authorities that no arrest has been made.

Homeland Security Secretary Janet Napolitano says the FBI wants to speak with individuals seen in at least one video from the Boston Marathon but says she isn’t calling them suspects.

Without providing details of the men’s appearance or what the video shows, Napolitano told the House Homeland Security Committee on Thursday that “there is some video that raised the question” of individuals the FBI would like to interview.

Napolitano said it’s still unclear whether the bombs that exploded near the finish line of the Boston Marathon were the work of foreign or domestic terrorists. She said the investigation is continuing “apace.”

Boston City Council President Stephen Murphy, who said he was briefed by Boston police, said investigators saw the image on surveillance footage they got from a department store near the finish line and matched the findings with witness descriptions of someone leaving the scene. “I know it’s very active and very fluid right now – that they are on the chase,” Murphy said. He added: “They may be on the verge of arresting someone, and that’s good.”

At an interfaith service honoring the victims Thursday, in Cathedral of the Holy Cross in Boston's South End neighborhood, President Barack Obama said "there is a piece of Boston in me" as he paid tribute to the city shaken by what he has called an act of terror.

"Every one of us stands with you," he said, declaring later: "You will run again!"

The second full day of the investigation Wednesday into the attack that killed three people and injured at least 176 brought jitters, rumors and at least the hope that investigators had made important progress, though Gov. Deval Patrick said that although the probe is “making some progress . . . it’s going to be slow, it’s going to be methodical.”

Page 2 of 2 - Boston’s federal courthouse, where hundreds had gathered in response to false reports of an arrest, was briefly evacuated because of a bomb threat. Officials also evacuated a Boston hospital, Brigham and Women’s, and Oklahoma City’s city hall because of suspicious vehicles outside. No explosives were found in those cases.

In Boston, many of the injured were released from hospitals. At Brigham and Women’s, which initially treated 35 people, only 11 were still hospitalized Wednesday evening, four of them in critical condition. Officials at all the hospitals said all of the critically wounded are now expected to survive.

Wednesday’s whirlpool of reports demonstrated the extraordinary promise and power that new technologies bring to criminal investigations, but also the risk and unreasonable expectations that now permeate such probes. When federal authorities asked the public for help Monday, they received thousands of video clips and still images of the bomb site.

Some people, empowered by smartphones and ever more sophisticated technology, didn’t leave the detective work to the professionals. They joined forces on sites such as Reddit.com to examine crowd pictures, searching for – and then virally distributing – image of backpacks that resembled the shredded bag in photos the FBI released Tuesday.

Black backpacks turn out to be ubiquitous, and when five of them were found in a single photo of the crowd on Boylston Street, the search quickly drew criticism from readers worried that innocent people could be harmed by being identified as suspicious. Others questioned whether black backpacks were even the most important lead, recalling the search for white box trucks that steered investigators astray in the Washington sniper case a decade ago.

At midday, news organizations such as CNN and the Associated Press reported that investigators had identified a suspect or made an arrest, leading the FBI to issue an unusual appeal: “Contrary to widespread reporting, there have been no arrests made in connection with the Boston Marathon attack. . . . Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting.”

CNN said it “had three credible sources on both local and federal levels” for its reports. “As soon as our sources came to us with new information, we adjusted our reporting.”

In Boston, police barricades still ringed a 12-block area around the finish line, guarded by city police and the Massachusetts National Guard.